This invention relates to franking of mail items and in particular to providing security for franking impressions printed on mail items.
Known franking machines utilise a print drum carrying settable print wheels to print franking impressions on mail items. The print drum carries a print die to print fixed information in the franking impression which usually comprises a fixed pattern as determined by the postal authority together with a franking meter licence number and the postal area from which the franked mail is dispatched. A further die may be provided to enable the printing of a slogan, for example advertising material relating to the sender of the mail item. The print wheels carried by the print drum are utilised to print a value of postage charge and the date on which the mail item is franked. The print wheels have type characters on their peripheries and are rotated relative to the print drum to bring a selected type character into printing position. Rotation of the print wheels used to print the postage value is effected by means of mechanisms controlled by electronic control and accounting circuits so as to set the print wheels to print a desired postage charge which has been entered by the user into the franking meter, for example by operation of thumb wheels or keys of a keyboard. It will be appreciated that, in order to prevent fraudulent printing of franking impressions, it is essential to ensure that the print wheels are able to print only the value of postage charge which has been entered properly into the franking meter and for which proper accounting has been effected by the accounting circuit of the meter. Furthermore it is necessary to permit only a single rotation of the print drum for each postage value accounted for by the accounting circuit and to prevent rotation of the print drum which could be used to print multiple franking impressions on mail items for which no accounting has been effected. With a mechanical printing arrangement such as a print drum and print wheels security of printing of franking impressions can be effected relatively easily by means of appropriate construction of mechanisms for setting the print wheels and retaining them in their set positions during rotation of the print drum and for preventing rotation of the print drum except when permitted by the accounting and control circuits. These mechanisms are housed together with the accounting and control circuits in a secure housing which prevents unauthorised tampering with the printing devices.
However such mechanisms for printing franking impressions with selected postage values are relatively complex and expensive to manufacture. Accordingly it has been proposed to use digital techniques for printing the franking impression. In such digital techniques a plurality of printing elements, such as ink jet or thermal ink transfer, are disposed in a matrix or a line and are operated selectively so as in combination to print a complete franking impression. When the printing elements are disposed in a line, the elements are operated selectively and repeatedly while the mail item is fed past the line of elements to progressively build up a complete franking impression. Digital printing devices are operated by electrical signals generated by the accounting and control circuits and this poses problems in ensuring adequate security in printing franking impressions because it is difficult to prevent false electrical signals being applied to operate the printing elements to print a franking impression with a value of postage charge for which proper accounting has not been effected.